Toward the Metabolic Network Reconstruction for Hetercystous Cyanobacteria

Cyanobacteria are one of the earliest biological lineages on earth. They are the only phylum of prokaryotes capable of oxygenic photosynthesis, and are the progenitors of the plant chloroplasts. The photosynthetic conversion of carbon dioxide to oxygen created the present atmospheric composition and has had an impact on all life on earth. However, huge amount of carbon dioxide, a prominent greenhouse gas, has been generated since the Industrial Revolution by diverse human activities, mainly through the combustion of fossil fuels. This is becoming one of the biggest challenges to not only human beings but also the entire biosphere, due to the derived environmental impact (e.g. climate change) to the ecosystems. Cyanobacteria are promising candidates to tackle this dilemma, because they have been fixing the carbon dioxide on this planet for billions of years.

Heterocyst-forming cyanobacteria are multicellular organisms in filamentous form. They can produce gas vesicles for buoyancy regulation and develop heterocyst cells for nitrogen fixation under nitrogen limitation. These features make them among the most autotrophic organisms and thus ideal candidates of metabolic engineering for solar energy utilization, carbon emission reduction, and biofuel production. However, current attempts for heterocyst-forming cyanobacteria engineering applications are severely limited by a lack of knowledge on their metabolic capabilities.

Previously, genome-scale metabolic network reconstructions have been focused on unicellular cyanobacteria, such as Synechocystis sp. PCC 6803 and Cyanothece strains, which are unable to fix nitrogen. Toward the metabolic network reconstruction for hetercystous cyanobacteria, I cataloged 34,378 genes encoding enzymes that catalyze 2,103 distinct metabolic reactions from 129 pathways, by examining 33 genomes of heterocyst-forming cyanobacterial strains. A major fraction of the discovered enzymes (56%) are found to be involved in the pathways for metabolism of carbohydrate, energy and amino acids. This information would serve as a basis for the genome-scale metabolic reconstruction for heterocystous cyanobacteria, and future metabolic engineering applications in reducing carbon emissions, or production of biofuels and other valuable natural products.